Method of refining lead



r more expedient treatment for removal of imiojreceptacle in which themetal is Patented Get. 15, 1929 PATENT foF cs JOHN P. WALKER, OFHAMMOND, INDIANA METHOD OF REFINING LEAD No Drawing.

The invention relates to the refining of lead, and particularly to theremoval of impurities from lead bullion.

An object of the invention is to provide a purities from unrefined leadthan that afforded by the standard pyrometallurgical softening practice.i v

The treatment may be given in the same melted, and- ,completed in lesstime than is required by standard practice. This treatment in the samereceptacle, for instance the kettle,permits saving of initialinstallation, or additional installation in case of desired increasedtion comprises softening capacity. Other objects and advantages of theinvention will appear from the body of tliespecification.

Briefly, the method of the present invencontacting the lead while inmolten state with a hydrated compound which is infusible, orsubstantiall so, at the temperature of the bath. The hy rated compoundemployed is preferably hydrated lime and preferably also the molten bodyof metal skilled in the art of softening or improvfn is stronglyagitated during the treatment.

The method may effectively be carried out by covering the surface ofmolten lead' in a suitable kettle of a type well known to those g lead,with the hydrated lime or other by drated compound, and forcing a streamof air or steam through the body of molten metal. The air or steam mayconveniently be introduced through pipes submerged in the molten lead,or mechanical agitation may be employed, being preferably effected bymeans of the well known Howard mixer, or an equivalent agitator.

. It has also been found that the, improvement hereinafter describedwill reduce, for example, 32% arsenic and 1.1% antimony, contained in100 tons of lead bullion, to 019% arsenic and .07 5% antimony in aperiod of thirteen hours.

It has also been found that this preliminary refining has a distinctadvantage in reducing the time required in the refining operation,- asperformed with softener furnaces, particularly in accelerating theremoval of arsenic Application filed August 25, 1928, Serial No.302,153.

or antimony when present in large amounts. For example, it has beenfound that when the impure lead bullion contains more than 2% arsenicand 1.3% antimony, 7 2 hours or more are required to remove theseimpurities from 200 tons by the standard softening practice. lVhen,however, bullion containing such excessive amounts of impurities issubjected to the method of the present invention, which may require from512 hours, the sub sequent softening operation can be accomplished in 18hours or even less time.

The efficiency of the process is by no means confined to the removal ofarsenic and antimony. In actual plant application, the addition ofhydrated lime at'the stage of removal of zinc from metal which has beenpreviously desilverized with zinc, gives an intense reaction, and uponbrisk stirring of the reagentinto the metal the surface of the bathbreaks spontaneously into flame. This application speedily removes theabsorbed zinc from the metal.

I will now describe 'an embodiment of the invention Which has proved tobe efiective and practical, but it is to course that I do not therebyintend to limit myself to the particular materials, proportions, etc.;the same being given merely by way of example and for purposes ofillustra tion only.

Lead bullion containing arsenic or antibe understood of mony is chargedinto a melting kettle of, say,

100 tons capacity and is melted by heat applied to the under surface ofthe kettle. The molten lead is heated to approximately 117 5 F. andmaintained at this temperature during the treatment.

When the lead has become molten it is covered with a layerof 300-500pounds of hydrated lime. The body of molten metal is stirred with aHoward mixer or equivalent device, or a stream of air or steam ispassed,

or a number of streams of air or steam are passed, through the bath, forexample by means of submerged, pipes. The pipes are conveniently securedto the 1111181 s1de of the kettle and extend down into the molten leadtwo-thirds of its depth or farther.

The treatment is continued generally for six hours or more, dependingon, the impurity content. The powdered lime becomes lumpy or isagglomerated by the cementing action of uncombined oxides. These lumpsare colored a light brown to yellow.

During the period of removal of arsenic and antimony a certain amount oflead is continuously oxidized to litharge and enters gold, etc., bymethods well knownto those skilled in the art. When the lead isdesilverized by the Parkes process using metallic zinc, the residualzinc absorbed by the lead may be speedily removed by the addition oflime followed by agitation with air or steam or with a mixer alone. Thisoperation may be carried out preferably in another kettle than the oneused for desilverizing. ,In case of unusual quantities of impurities thetreatinent may of course be repeated, using fresh ime. 1

The method is also efliciently applicable in case the arsenic is presentin the unrefined bullion in excess of 2% and the major softening iscarried out with furnaces. In this case thetreatment is preferablycontinued only until the arsenic content is reduced to 2%, since theantimony is not removed until the greater part of the arsenic has been"eliminated. However, all of the arsenic, and antimony can be removed bythe method if desired, viz, by prolonging the time of treatment and ifnecessary increasing the number of fresh additions of hydrated lime.Arsenic and antimony can be removed by treatment with lime in a'shorterperiod than is required for the same operation when carried out in theusual way in a standard softening furnace. The Weight of skim producedper unit of antimony removed is less in the treatment 'by the presentmethod than in the usual furnace softening.

As indicated above, the operating factorsare somewhat variable.Temperatures as low as 800 F or perhaps a little lower, may be employedfor zinc removal, but it has been found that the best elimination ofarsenic and antimony is obtained when the method is carried out at atemperature of 1100 F. or higher.

The purpose of stirring the molten bullion is to bring all of the leadinto contact with the lime .and to turn the metalover rapidly so that itwill come in contact with the lime a number of times during the courseof the tact. Other methods of stirring, such for example as byover-pumping, may of course be employed.

Hydrated compounds other than hydratedlime may be employed. Magnesiumcompounds may be substituted for the lime compounds. Hydrated lime,however; possesses the advantage of being finely divided and requires nopreliminary treatment. also inexpensive, costing at the present timeless than five cents per ton of lead treated. Experience provides, ofcourse, the best indication of the percentage. of hydrated material tobe employed, but tests so far made indicate that from about 0.2 to' 0.6%by A weight is suitable, and highly satisfactory elimination is obtainedwhen an amount of hydrated lime equal to about 0.4% of the weight of thelead is employed, with bullion of average impurity.

As examples of the efliciency of the invention, plant experience hasshown that the removal of arsenic from bullion when using hydrated lime,according to the process as described, is accomplished in less than halfthe time required for removal when using air alone. Also that a bullionwhich requires 36 hours forsatisfactory removal of antimony in a2'00-ton softener, requires only 12 t6 18 hours when treated withhydrated lime in two kettles of 100 tons capacity each.

In the removal of zinc fromidesilverized lead, the reaction of the zincwith the hydrate is so intense that the hydrogen liberated bursts intoflame spontaneously;

The active radical, or portion of the hydrate, is the combined water,which is liberated in the nascent state. Steam, the use of Which forremoving zinc from desilverized lead is well known to the art, does notact so intensely because it is added as such, and is not a productof-the reaction.

- The chemical reactions when rising hydrated lime, as described in theprocess, I

belie; to be essentially as follows:

1. emoving zinc from desilverized lead:

Further oxidation of arsenic and antimony and combination with limeproduces arseniates and flIltHDOIllfitCS.

The hydrogen, which would be expected.

to. reverse the reactions, is liberated at the free surface of the metaland is immediately removed from the zone of reaction, by virtue of itsextremely low density, and the porous solid nature of the hydrated limecovering.

This solid nature of the reagent permittingthe free passage of hydrogenfrom the zone of reaction is an advantage of the process over the use ofbasic alkalles, such as caustic soda. Other advantages of the solidskims produced by hydrated lime over those resulting from use of causticalkalies are as follows 1. They can be treated by common furnacepractice, Without any additional chemical plant as required forregenerating or restoring expensive alkalies.

2. The separation of the skims from the remaining metal is moreconvenient.

3. The skims are far safer to handle. Any

moist tools applied to molten caustic alkali used for refining by meansof air'or steamv alone.

Relative to the character of the lime dross, ordinary softener furnaceskims high in arsenic are commonly supposed to carry the arsenic as leadarsenate. Experience has shown that an excessive amount of coal isrequired to reduce this lead arsenate in the residue furnace and leave aslag of required lead content. When the treatment according to thepresent invention is employed in kettles, the coal required forreduction in the residue furnacewhen treating the lime dross is normal.This would indicate that the arsenic in the lime dross is combined withthe lime,- allowing the lead oxide as such to be easily reduced by coal.Herein therefore lies another articular advantage of the method of reing in accordance with the present invention.

The lime due furnace with the other refinery by-prod ucts normallytreated in this furnace, or it may be treated in a, blast furnace. Iftreated dross may be treated in the resicontacting lead in the moltenstate with a basic hydrated compound which is infusible at thetemperature of the lead bath.

3. Method of refining lead which'comprises contacting lead in the moltenstate with hydrated limel 4. Method of refining lead which comprisescontacting lead in the molten state with a hydrated compound which isinfusible at the temperature of the lead bath and agitating the body ofthe metal.

5. Method of refining lead which comprises contacting lead in the moltenstate with hy: drated lime and agitating the body of the metal.

6. Method of refining lead which comprises contacting lead'in the moltenstate with hydrated lime' and agitating the body of the metal with astream of gaseous fluid.

7. Method of refining lead which comprises contacting lead in. themolten state with hydrated lime and agitating the body of the metal witha stream of air.

8. Method of refining lead bullion which comprises heating the same to atemperature of from about 800 F. to about 1200 F mixing hydrated limewith the same and agitating the body of metal during the treatment.

9. Method of refining lead bullion which com rises heating the same to atemperature of trom' about 800 F. to about 1200 F., mixing hydrated limewith the same and agitating the body of metal during the treatment witha stream of air.

10. Method of refining lead bullion which comprises heating the same toabout 117 5 F., mixing therewith from two tenths to sixtenths percent byweight of hydrated lime and agitating the body of metal duringthetreatment. i Y

11. Method of refining lead buliion which comprises heating the same toabout 117 5 F., mixing therewith from two-tenths to sixtenthspercent byweight of hydrated lime and agitating the body of metal during thetreatment with a stream of air.

In testimony-whereof, I aflix my signature.

- q JOHN P. WALKER.

in the latter, the charge carries only'the amount of coke required tomaintain the tem perature necessary for. a liquid slag. This 1. Methodof refining lead which comprises I contacting lead in the moltenstatewitha hydrated compound which is infusible at the temperature ofthe lead bath.

2. Method of refining lead which comprises

